1. Field of Invention
This invention relates to pulse position detection systems, methods and program products. More particularly, the invention relates to amplitude-offset invariant template detection for pulse position estimation: methods, systems and program products.
2. Description of Prior Art
The operation of many discrete systems is based on analysis of a signal in order to find the existence and position (in time) of a predetermined shape. Laser-scanner decoding, mobile ranging, and digital signal processing for feature extraction all depend on knowledge of the position of particular shapes within a signal, not merely their existence. This type of positional analysis is useful in any application where an output or intermediate stage is designed to produce an amplitude-varying signal whose “pulses” correspond to particular events of importance. Determination of the position of said “pulses” (and hence, of the monitored events) may be accomplished by a variety of methods, including comparison of the amplitude to some set threshold and the novel approach presented in “Spread Spectrum Indoor Geolocation” by B. Peterson, C. Kmiecik, R. Hartnett, P. Thompson, and J. Mendoza published in the Journal of the Institute of Navigation, Vol. 45, No. 2, Summer 1998 (reference 1)
The accuracy of many pulse-position detection systems, however, becomes suspect in the presence of noise corruption and multipath impairments (the random combination of multiple, time-shifted versions of the signal). Though there are several techniques available to combat multipath impairments, the template cross-correlation method has the distinct advantage of being inherently resistive to such degradation, as illustrated in ref. 1 This technique, though robust against noise corruption and multipath, suffers from poor accuracy in analyzing vertically shifted signals (shifts with respect to the amplitude axis, not time).
One practical example where this inherent weakness affects system performance is amplitude offset caused by multipath in direct sequence code division multiple access (DS-CDMA) systems, illustrated in FIGS. 1A, B and C where signal amplitude (A) is shown as a function of time (t). The output of a DS-CDMA correlator, a component of the typical receiver structure, is a signal 100 consisting of pulses 102 interspersed with relatively low signal levels. The level of these intermediate points is a function of the spreading sequence used—specifically, of the autocorrelation function of the sequence. It is common for the spreading sequence's autocorrelation to appear similar to the correlation signal shown in FIG. 1A, where the signal level 104 between pulses is not 0, but maintains a relatively large magnitude compared to that of the pulse, itself. Large magnitudes between pulses are typically associated with short spreading sequences and partial correlations (the procedure of treating a large spreading sequence as groups of smaller sequences, usually split along bit-scale divisions, in order to simplify the correlation procedure at the cost of weakened autocorrelation properties). When more than one such signal 106 is combined, with random time shifts (the case when transmitted in many wireless environments), the effect is an amplitude shifting of the peaks of the signals—even if no actual “distortion” of the peaks occur. The sum of FIGS. 1A and B creates an offset 108, shifting of the signal below the time axis, as shown in FIG. 1C.
Though this vertical shift 108 has no effect on the position (in time) of the signal pulses, the template cross-correlation is affected as shown in FIGS. 2A, B and C where the template 200 (dark line) correlates best to the signal 102 (dotted line) at different points because of offset 202 or vertical signal shift. Because the template's vertical offset 204 is static, rather than correlating best with the signal 102 at the point where the peak 206 actually occurred, the template correlates best at different positions, depending, not on the time axis, but on the vertical position of the signal. Additionally, because the template cross-correlation method requires that the signal be normalized prior to template correlation (in order to counteract any effects from the signal being modulated), vertical offsets can cause the system to use incorrect normalization factors—distorting the shape of the signal as will be described hereinafter in FIGS. 5A, B, and C. Multipath, therefore, affects the horizontal position estimate, not only by distorting the pulse shape, but also through vertical shifting of the signal. The tolerability of this effect depends on the sharpness of the pulses (typically “dulled” in practical systems by low-pass filtering) and the accuracy desired in the system.
Having shown that template cross-correlation is sensitive to amplitude offsets and that there is at least one practical application where this weakness can cause system inaccuracy, the need for an amplitude-offset invariant template detection method has been established.
Prior art related to signal detection via template cross-correlation includes:
(1) U.S. Pat. No. 5,910,905 issued Jun. 8, 1999 discloses detecting the presence of dispersed broadband signals in real time. The present invention utilizes a bank of matched filters for detecting the received dispersed broadband signals. Each matched filter uses a respective robust time template that has been designed to approximate the dispersed broadband signals of interest, and each time template varies across a spectrum of possible dispersed broadband signal time templates. The received dispersed broadband signal x(t) is received by each of the matched filters, and if one or more matches occurs, then the received data is determined to have signal data of interest. This signal data can then be analyzed and/or transmitted to Earth for analysis, as desired. The system and method of the present invention will prove extremely useful in many fields, including satellite communications, plasma physics, and interstellar research. The varying time templates used in the bank of matched filters are determined as follows. The robust time domain template is assumed to take the form w(t)=A(t) cos {2.phi.(t)}. Since the instantaneous frequency f(t) is known to be equal to the derivative of the phase .phi.(t), the trajectory of a joint time-frequency representation of x(t) is used as an approximation of .phi.′(t).
(2) U.S. Pat. No. 6,342,854 issued Jan. 29, 2002 discloses a position determining system, for receiving digital telephone signals transmitted by a number of transmission sources BTS. The system has a pair of receiving stations, CBU and CRU, one at a known position O and another on a roving object R; a position determining processor CPP; and means for passing a link signal, L1 and L2, from each of the receiving stations to the position determining processor, the link signal containing information about the signals received at the receiving station from the transmission sources. Each of the receiving stations is arranged to receive the signals from the respective transmission sources substantially simultaneously. The position determining processor is arranged to compare the information received from the one receiving station with the information received at the other receiving station, and to determine the time delay between the respective signals received at both receiving stations in order to determine the position of the roving object.
None of the prior art discloses or suggests correlating a received signal with a template of the transmitted signal to determine and correct modulation and offset effects in the received signal thereby obtaining a more accurate pulse position detection. Similarly, none of the prior art provides solutions meeting the particularly stringent accuracy requirements necessary for real-time location tracking in the IEEE 802.11 local-area-network signal environment.